1. Field of the Invention
The present invention relates to a fixing device, a fixing method, an image forming apparatus, and an image forming method.
2. Description of the Related Art
A known fixing device is arranged in an image forming apparatus that uses electro-photographic technology. The fixing device fixes a toner image that is electro-statically supported on a sheet by means of pressure and heat, and includes a fixing roller and a press roller that contacts and exerts pressure on the fixing roller. A heating device, including a heater, provides heat to at least one of the fixing roller and the press roller. Accordingly, a nipping area where the fixing roller and the press roller contact each other is heated, and the toner image formed on the sheet that passes through the nipping area is fixed on the sheet.
The fixing device includes a temperature detecting device that is arranged, for example, on the central part of the rotation axis of the fixing roller. When a sheet passes through the nipping area, the sheet draws heat from the fixing roller. Thus, power from the heater must be controlled to compensate for the heat drawn by the passing sheet. In this manner, the temperature of the nipping area is kept at a predetermined temperature.
In this known configuration, a problem exists in that the temperature of edges of the fixing roller gradually increase when many sheets continuously pass through the nipping area, because the sheets do not contact the edges of the fixing roller and, thus, do not draw heat from the edges of the fixing roller. When the amount of heat around the edges of the fixing roller is set to be lower to avoid this problem, the temperature of the edges of the fixing roller can become too low during the standby time of the fixing device. During standby, the heat of the edges of the fixing roller is transmitted to side walls through bearings of the fixing roller and through a gear fixed to the frame of the fixing roller. Due to the low edge temperature, the portions of a toner image positioned on the parts of a sheet that contact the area around the edges of the fixing roller are not fixed well.
Japanese Patent Publication No. 05-134575 (“JP '575”) describes a fixing device designed to address this problem. The fixing device is arranged in a copier and includes a fixing roller and a press roller that contacts and exerts pressure on the fixing roller, thereby forming a nipping area between these two rollers. The fixing device also includes a temperature control device. A first heater and a second heater, each of them being a halogen heater, are arranged in the fixing roller. The temperature control device detects the temperature of the surface of the fixing roller. The first heater can be controlled such that that heat in the portion of the fixing roller surface on which the sheet does not pass is lower than heat in other portions of the fixing roller surface. The second heater can be controlled such that heat in the portion of the fixing roller surface on which the sheet does not pass is higher than heat in other portions of the fixing roller surface.
In the fixing device of JP '575, electric power is supplied to the first heater and the second heater when the power of the copier is ON. When the temperature control device detects that the temperature of the surface of the fixing roller reaches a predetermined temperature, as a result of receiving heat from both the first heater and the second heater, the first heater is turned off and only the second heater continues to heat the fixing roller. The copier remains in this standby state until a sheet passes through the nipping area of the fixing device.
When a copy button of the copier is pressed, the second heater is turned off while the first heater is turned on. During a copying operation, only the first heater heats the fixing roller and controls the temperature of the fixing roller such that the temperature is kept a predetermined level. After the coping operation finishes, the copier reverts to the standby state and the fixing roller is heated only by the second heater.
However, the fixing device of JP '575 suffers from several problems, especially when a fixing belt is arranged around the fixing roller, and when the nipping area is formed between the fixing belt and the press roller.
For example, when the copier is in the standby state, the temperatures of the edge of the fixing roller and the press roller generally become lower, because heat from the fixing roller and the press roller is transmitted to side walls through bearings that support these rollers, as described above. When a heat roller and the fixing belt are made of materials having high heat conduction, heat from the press roller and the fixing roller is radiated through the heat roller and the fixing belt.
Accordingly, when fixing is started, the temperature of the edges of the fixing belt is lower than is necessary to properly fix a toner image to areas of a sheet contacting the edges of the fixing belt.
In order to avoid the problem, the heat quantity of the heater in the part that heats the edge of the press roller is set higher. The temperature of the part of the fixing belt, which a sheet contacts when the sheet passes through the nipping area, does not become too high because the sheet draws heat from the fixing belt. However, the temperature of the edge of the fixing belt, which contacts the outer edges of passing sheets, becomes too high with continuous sheet passing.
In addition, the wattage of the heater in the press roller must be continuously increased to maintain the temperature of the center of the nipping area in a range that is enough for fixing, even when sheets continuously pass through the nipping area. Therefore, the fixing device consumes much electricity during operation.